In the electronic industrial field, wire bonding, tape automated bonding (TAB) and flip chip techniques are used for bonding a chip on a circuit board. Among these techniques, flip chip technique will be the most popular in the future because the number of contacts in every chip may achieve 16000 in this technique which is more than that in the other two techniques.
In the flip chip technique, a solder bump on an aluminum pad is utilized to bond a chip on a board. However, the solder bump is difficult to be deposited on the aluminum pad because the surface of the aluminum pad is subject to oxidation and thus an oxide layer is usually formed thereon. In order to overcome this problem, a conventional method uses evaporation deposition or electroplating procedure to produce the solder bump. Before the evaporation deposition or electroplating procedure, several metal layers, e.g. Cr/Cu/Au or Ti/Ni/Au, have to be sputtered on the aluminum pad first. The chromium layer serves as an adhesive layer because of its good adhesion property with the passivation layer, the copper layer functions for moistening, and the gold layer is used to prevent the oxidation of copper and enhance the solderability.
The conventional method has the shortcomings listed below.
1. Both evaporation deposition and electroplating methods have to be executed in vacuum which results in a complicated process; PA0 2. A mask is necessary for preventing the metal from being deposited or plated on the passivation layer; and PA0 3. In the evaporation deposition process, the vapor pressure of lead is higher than that of tin so that lead vaporizes and deposits on the aluminum pad earlier than tin, and thus the component of solder will not be uniform; and PA0 4. The solder bump obtained by the conventional method is flat and difficult to be bonded to the board. Therefore, the chip with the solder bump has to be heated in a reducing atmosphere to have the solder melt and molded to a proper shape for bonding. That will result in a more complicated process. PA0 1. M. Inaba, K. Yamakawa, and N. Iwase, "Solder Bump Formation Using Electroless Plating and Ultrasonic Soldering", IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. 13, No. 1, 119, March 1990. PA0 2. K. Wang, K. Chi and A. Rangappan, "Application of Electroless Nickel Plating in the Semiconductor Microcircuit Industry", Plating and Surface Finishing, 70, July 1988.
In addition to the method for depositing a solder bump, The selection of materials for an adhesive layer and/or diffusion barrier layer is important as well. The adhesivity to the aluminum pad, the reaction rate with aluminum or Pb-Sn alloy, and the atomic diffusivity of the adhesive layer and/or diffusion barrier layer should all be taken into account. In general, the grain boundary diffusion of a metal film is more significant than its volume diffusion at a low temperature so that the final device is subject to be rejected owing to the grain boundary diffusion. Therefore, a single crystal or an amorphous material is usually used to construct the diffusion barrier layer to avoid the grain boundary diffusion. As the electroless Ni-P is amorphous, it will not be deposited on the inert material, i.e. the substrate, so that a mask is not needed therefor, and moreover, it has a good adhesiveness to the aluminum pad. Therefore, an electroless Ni-P layer is a good choice for serving as a diffusion barrier layer, as disclosed in Ref. 1.
In 1988, Wong et al disclose a method for producing a barrier layer which is an electroless Ni-P layer and a solder bump which consists of 60% Sn and 40% Pb, as shown in Ref. 2. However, the melting point of the solder bump Wang et al produces is too low (about 188.degree. C.) to be extensively applied to the industry. In the electronic package technique, "chip to substrate" belongs to the first level package and "substrate to card" belongs to the second level package. The higher the package level, the lower should the melting point of the material be to prevent the bonded material from melting again. Therefore, a solder bump having a higher melting point should be chosen for this purpose. In addition, the process disclosed by Wang et al needs "double" zincating procedures to form a zinc layer before forming the electroless Ni-P so that the process is complicated. On the other hand, a flux suitable for the high melting point bump should be carefully selected. Rosin which is a combination of three isomers of diterpene acid, i.e. sylvic acid, d-pimaric acid and l-pimaric acid, is conventionally used as a flux in the electronic industry. The sylvic acid is easy to proceed rearrangement at about 300.degree. C. to form a mixture of n-eoabietic acid and pyroabietic acid which are chemically inert. Therefore, rosin is not suitable for manufacturing a high melting point bump.